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(class Mammalia, subclass Prototheria)

An order comprising the duck-billed platypus (Ornithorhynchus anatinus, see Ornithorhynchidae) and the echidnas or spiny ant-eaters (Tachyglossus and Zaglossus, see Tachyglossidae). There were some extinct forms, of which very few are known in detail, though of those some attained large sizes. The echidnas have no fossil record older than the Pleistocene, but a fossil platypus, Obdurodon, is known from the Miocene and in the early 1990s teeth of an undoubted platypus-like form, Monotrematum sudamericanum, were discovered in Palaeocene deposits in Patagonia. Two Cretaceous genera, Steropodon and Kollikodon, are also known. In view of their reptilian affinities they are thought to represent a separate and direct line of descent from the earliest Mesozoic animals, possibly the Docodonta, independent of the line leading to other mammals, but the dental features of Steropodon are considered by some to point to affinities with the Eupantotheria. They retain many primitive features and are quite unlike marsupials or eutherians. The rectum and urinogenital system open to a common cloaca (the name ‘monotreme’ is derived from the Greek monos meaning ‘alone’ and trema meaning ‘hole’, although the feature is shared by marsupials and some insectivores). The male is heterogametic as in other mammals. The young are hatched from large, yolky eggs, incubated in a nest by the female platypus and in a pouch in the echidnas. The embryos develop a caruncle and egg-tooth. After hatching they are fed milk secreted by the female from specialized sweat glands which do not open through central nipples. The diaphragm is fully developed, and the heart possesses a single left aortic arch as in other mammals. The larynx is developed, and monotremes make sounds. The tarsus of the male has a grooved erectile spine to which (in the platypus) poison is fed from a gland in the thigh. The poison (said to be capable of killing a dog) may serve to immobilize the female during mating. Adults lack teeth but possess bills. The skull is specialized but retains primitive features. The jaw consists of a single bone. The cervical ribs are not fused, the shoulder girdle is reptilian in form, the pelvis is reminiscent of that of marsupials. The body is covered with hair, in the echidnas partly modified into spines on the back. The limbs are modified for digging (echidna) or swimming (platypus); it has recently been discovered that both platypus and echidna locate their prey by detecting weak electrical fields around the snout. Echidnas hibernate, and in hot weather all monotremes shelter in burrows or caves. Monotremes are known only from Australia and New Guinea, where they may have survived because of their high degree of specialization and the isolation of that continent.